K-type thermocouple is a kind of temperature sensor. K-type thermocouple is usually used with display instrument, recording instrument and electronic regulator. K-type thermocouple is usually composed of main components such as temperature sensing element, installation fixture and junction box.
The K-type thermocouple is used as a temperature sensor. The K-type thermocouple is usually used with display instruments, recording instruments and electronic regulators. The K-type thermocouple can directly measure the surface temperature of liquid vapor, gaseous media and solids in various production ranges from 0℃ to 1300℃.
K-type thermocouple is the cheapest metal thermocouple with the largest consumption at present, and its consumption is the sum of other thermocouples. The diameter of K-type thermocouple wire is generally 1.2mm～4.0mm.
The nominal chemical composition of the positive electrode (KP) is: Ni:Cr=90:10, the nominal chemical composition of the negative electrode (KN) is: Ni:Si=97:3, and its service temperature is -200℃~1300℃.
The K-type thermocouple has the advantages of good linearity, high thermoelectromotive force, high sensitivity, good stability and uniformity, strong oxidation resistance, and low price. It can be used in oxidizing inert atmosphere and widely used by users.
K-type thermocouples cannot be directly used in sulfur, reducing or reducing, oxidizing alternate atmospheres and vacuum at high temperatures, nor are they recommended for use in weakly oxidizing atmospheres.
Principle of k-type thermocouple probe
It is converted into a thermoelectromotive force signal and converted into the temperature of the measured medium through an electrical instrument (secondary instrument). The basic principle of thermocouple temperature measurement is that two conductors of different components form a closed loop. When there is a temperature gradient at both ends, current will flow through the loop. At this time, there is an electromotive force-thermoelectromotive force between the two ends. This is the so-called Seebeck effect. Two homogeneous conductors with different compositions are thermoelectrodes, the end with a higher temperature is the working end, the end with a lower temperature is the free end, and the free end is usually at a certain constant temperature. According to the functional relationship between thermoelectromotive force and temperature, a thermocouple index table is made; the index table is obtained when the free end temperature is at 0℃, and different thermocouples have different index tables.
When the third metal material is connected in the thermocouple circuit, as long as the temperature of the two junctions of the material is the same, the thermoelectric potential generated by the thermocouple will remain unchanged, that is, it will not be affected by the third metal in the circuit. Therefore, when the thermocouple measures the temperature, the measuring instrument can be connected, and the temperature of the measured medium can be known after the thermoelectromotive force is measured.
Features of k-type thermocouple
Detecting (temperature measurement) component The thermocouple is one of the most commonly used temperature detection components in the industry. Must be equipped with a secondary instrument, its advantages are:
①High measurement accuracy. Because the thermocouple is in direct contact with the measured object, it is not affected by the intermediate medium.
②Wide measuring range. Commonly used thermocouples can measure continuously from -50 to +1600°C, and some special thermocouples can measure as low as -269°C (such as gold, iron, nickel and chromium), and up to +2800°C (such as tungsten-rhenium).
③Simple structure and convenient use. Thermocouples are usually composed of two different metal wires, and are not limited by size and beginning, and there is a protective sleeve outside, which is very convenient to use.
2 According to the temperature measurement range and accuracy, select the thermocouple with the corresponding index number
When the operating temperature is 1300~1800℃ and the accuracy is relatively high, the B-type thermocouple is generally used; the accuracy is not high, and the atmosphere allows the use of tungsten rhenium thermocouples. If the temperature is higher than 1800℃, tungsten rhenium thermocouples are generally used; 1000~1300℃ requires high accuracy and high accuracy. Available S-type thermocouple and N-type thermocouple; below 1000℃ generally use K-type thermocouple and N-type thermocouple, below 400℃ generally use E-type thermocouple; below 250℃ And the negative temperature measurement generally uses T-type thermocouple, which is stable and high precision at low temperature.
K-type thermocouple temperature measurement circuit
Thermocouple temperature measurement must be composed of three parts: thermocouple, connecting wire and display instrument. The figure below is the simplest schematic diagram of thermocouple temperature measurement.
According to the thermocouple core and thermocouple wire 1 composed according to the figure on the right, if the hot end of the thermocouple is heated so that the temperature at the cold and hot ends is different, a thermoelectric potential will be generated in the thermocouple circuit. This kind of physical The phenomenon is called thermoelectric phenomenon (ie thermoelectric effect). The electric potential generated in the thermocouple circuit is composed of two parts: the temperature difference electric potential and the contact electric potential. Contact potential: It is a thermoelectric potential generated when two conductors with different electron densities are in contact with each other. When two different conductors A and B are in contact, assuming that the electron densities of conductors A and B are Na and Nb and Na "Nb, respectively, on the contact surface of the two conductors, the electron diffusion rate in the two directions is not Similarly, the number of electrons diffused from conductor A to conductor B is greater than the number of electrons diffused from B to A. Conductor A loses electrons and becomes positively charged, while conductor B gains electrons and becomes negatively charged. Therefore, an electrostatic field from A to B is formed on the contact surface of the two conductors A and B. This electric field will hinder the continuation of the diffusion movement and accelerate the movement of electrons in the opposite direction, increasing the number of electrons from B to A. , And finally reach a state of dynamic equilibrium. At this time, a potential difference is also formed between A and B. This potential difference is called the contact potential. This potential is only related to the temperature of the contact point between the properties of the two conductors. When the materials of the two conductors are constant, the contact potential is only related to the temperature of the contact point. The higher the temperature, the more active the electrons in the conductor, the more electrons diffused from the A conductor to the B conductor, and the greater the electromotive force generated at the contact surface, that is, the greater the contact potential.
K-type thermocouple compensation wire
K-type thermocouple compensation wire use characteristics
1. The highest working temperature of the conductor: heat-resistant grade: poly ethylene propylene (F46) 200℃, soluble polytetra-ethylene 260℃, silicone rubber 180℃, ordinary grade: polyvinyl chloride 70℃ and 105℃, cross-linked polyethylene 90 ℃, low-smoke halogen-free flame-retardant polyolefin 70℃, cross-linked 90℃ and 125℃.
2. Low ambient temperature: PVC sheath: -40℃ for fixed laying, -15℃ for non-fixed laying, plastic insulation and sheath: -60℃ for fixed laying, -20℃ for non-fixed laying.
3. Allowable bending radius of cables: the minimum of non-armoured cables is 6 times the outer diameter, the minimum of copper tape shielded or steel tape armored point cables is 12 times the outer diameter of the cable, and the minimum plastic insulation and sheath is 10 times the outer diameter of the cable.
K-type thermocouple compensation wire is the most commonly used thermocouple. It is connected with the adjustment and display instruments, almost without exception, using copper-copper-nickel 40 compensation wire. This kind of compensation wire is cheap. However, the mechanical properties of the positive and negative pole materials are very different. The positive wire has extremely poor strength and is not resistant to flexing. Therefore, it is designed with a thicker wire diameter, which makes installation and use inconvenient. This is its insurmountable shortcoming. Generally, PVC insulated and sheathed compensation wires or cables have excellent moisture-proof, wear-resistant and flame-retardant properties; high temperature resistance * plastic insulated and sheathed compensation wires or cables adopt continuous extrusion process, which is more resistant to high temperature, cold, It is resistant to acid, alkali, oil and water, non-flammable, and anti-aging. The product quality is higher than the national standard GB/T4989-94 (equivalent to IEC584-3).